1. Field of the Invention
The invention relates to a force sensor for preferred application on the tip of an elongated device, and to a method for measuring a force and the effective direction thereof.
2. Description of Related Art
A particular case of application of this invention relates to catheter technology, which is defined by an elongated device for at least partial insertion into an organism through a body orifice. These elongated devices are used primarily in minimally invasive surgery and diagnosis on, in particular human, bodies. In order that, during the invasion of the elongated device, no body vessels are injured by a tip, in the vicinity of the body, of the elongated device, which is generally to be operated manually by the treating physician, the physician necessarily orients himself by what forces are conveyed to him at a handle of the elongated device. Owing to the friction and the inserted catheter/guide wire mass that increases continuously in the course of the invasion of the catheter or guide wire into the body, the force conveyed to the treating physician at the handle provides virtually no useful information about the forces actually occurring at the catheter/guide wire tip. In order that the treating physician can convey the correct actuating force to the handle of the catheter, this requires an extremely extensive wealth of experience in operating catheter/guide wires.
DE 103 03 270 A1 discloses a catheter arrangement in which the force acting on the catheter tip during insertion is measured. The corresponding force is conveyed to the physician in tactile fashion via a haptic handle. This makes it easier to find for example blood vessel junctions or perforations at the heart septum, especially for an inexperienced physician, and forms the basis for intuitive handling of the device. An electrodynamic drive device that serves for generating the haptic force prestress and uses a measurement variable representing the force of the tip is known from DE 103 19 081. The invention presented here permits the force measurement at the tip of the elongated device and the evaluation of the measurement signals, which is necessary for implementing the invention from DE 103 03 270 A1.
In accordance with U.S. Pat. No. 6,221,023 B1, a force sensor based on resistive functional operation is provided at the tip of catheters. The force introduced into the sensor is picked up by a resistance bridge circuit. The construction of this sensor involves a high outlay on account of the large number of parts. The associated manufacturing and assembly costs make the known sensor unsuitable particularly for catheters on account of the preferred disposable property thereof. Furthermore, the area for primary contact-connection of the measuring element is embodied perpendicular to the longitudinal direction of the catheter and the area for contact-connection is therefore limited by the diameter of the elongated device. Therefore, and owing to the high number of parts, the miniaturizability of the force sensor in particular below a catheter diameter of less than 3 mm can be realized only with an extremely high structural outlay. On account of the large dimensions it can be inferred that the sensor completely closes the catheter tube at the tip. This means, however, that the function of the catheter tube, through which instruments and fluids are introduced into the body, is no longer expediently provided. It is not possible to integrate the sensor into the significantly thinner guide wire required in catheterizations, on account of the large dimensions, the high number of parts and the unfavorable wire guidance through the contact area oriented perpendicular to the longitudinal direction.
JP 06190050 A describes a tactile sensor that can be fitted to the outer wall of catheters. It is proposed to produce this sensor from a very thin silicon wafer. However, very thin elements cannot pick up the forces of approximately 300 mN that occur. The use of thicker, stabler silicon wafers is ruled out since bending of thicker wafers, which is necessary for integration into the device, would lead to the material breaking apart.
“Beccai L et al.: Silicon-based three axial Force Sensor for Prosthetic Applications. Sensors and Microsystems, Proceedings of the 7th Italian Conference 2002” presents a force sensor that is intended to be integrated into leg prostheses. The force vector can be determined by measuring the force in terms of direction and amplitude. Resistive elements are used for measuring the forces, said resistive elements not being connected up to form a “Wheatstone bridge”. For integration into an elongated device, however, bridge interconnections should be striven for, whereby a high measuring accuracy can be obtained since the signals can then be transmitted virtually independently of lead resistances of the long, thin leads. Precisely in the case of long devices inserted into the body, the measurement signal is otherwise dependent on the temperature of the lead and thus on the insertion length of the device. The production of the elements requires an expensive multistage dry etching process on both sides. The measuring element nevertheless additionally has to be mounted over a special counter-body in order to enable the measuring beams to be deflected. The sensor therefore comprises at least two parts which have to be connected to one another exactly. Owing to the dictates of the construction, the exposed force pick-up, called a mesa, has a great length, namely approximately a length corresponding to the thickness of the starting material. Consequently, this sensor fundamentally always exhibits a measuring sensitivity that is higher by a multiple, but also susceptibility to destruction, relative to lateral forces, in comparison with the force acting in the longitudinal direction. That is unfavorable since it is desirable to measure particularly the forces in the longitudinal direction of the device or to obtain a balanced sensitivity for different force components. The measuring element has a plane of the electrical contacts that is perpendicular to the longitudinal direction, such that the contact-connection area is limited to the diameter of the installation area, which makes contact-connection more difficult. The measuring element has a large diameter of approximately 1 mm relative to guide wires, such that integration into the guide wire is not possible.
This shows that the prior art of tactile sensors for integration into catheters does not meet the requirements made with regard to miniaturization, high stability, simple production and low costs.